A photodetector is a device converting a photo signal into an electronic signal and extensively used in various fields, e.g. communication, computer engineering, controlling engineering and medical engineering etc. However, different photodetectors are needed for different uses, and also are needed to be altered according to the requirements of different photo signal frequencies. The detectable frequency band of conventional photodetectors is so narrow that a required photo response cannot be obtained when they are applied to a case needing a wide band or a frequency near an cut-off frequency of the used photodetector. This defect can be obviated by parallel connection of photodetectors with compensating frequency ranges.
The frequency range of a photodetector depends on a semiconductor material it uses. For example, silicon is used in a photodetector having a frequency range of 0.8.about.0.9 .mu.m, and a Ge, InGaAs or lnGaAsP is used in a photodetector having a detectable frequency range of 1.0.about.1.6 .mu.m, etc. The reason why a semiconductor has a frequency range of a photodetector is that a semiconductor material with a specific band gap accepts a specific frequency band. Therefore, if a certain frequency or wavelength is needed, a photodetector with a certain semiconductor material is used; and if a wide frequency range is needed, several photodetectors with different semiconductor materials are used.
The shortcomings of the conventional photodetectors are lacking of flexibility m circuitry, assembly and application, and having troubles m the match among the voltage, frequency and circuit.